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Integrated anisotropic rock physics model

a rock physics model and anisotropy technology, applied in the field of geological prospecting, can solve the problems of stress-induced anisotropy being more difficult to predict, differentiating the closure of soft pore or crack in the rock, and first-order theories of little practical us

Active Publication Date: 2010-03-09
EXXONMOBIL UPSTREAM RES CO
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Problems solved by technology

This differential compaction will result in a differential closure of soft pore, or cracks, in the rock.
Unlike shale anisotropy, stress-induced anisotropy is more difficult to predict because the principal stresses are controlled by many factors including overburden, tectonic, local structures, fault systems and rock properties.
This limitation makes the first-order theories of little practical use.
However, either scheme drastically slows down the numerical computation.
Furthermore, there is no model that can handle both shale anisotropy and stress-induced anisotropy simultaneously.
Despite some limited success of such models, there are increasing concerns about their applicability to seismic inversion (i.e., solving for petrophysical properties using seismic data) since they are data-driven.
Without a large amount of data to calibrate these empirical models, they often provide incorrect, sometimes even misleading, results.
In many exploration and / or development circumstances, one often does not have the data needed for the calibration.
Another major drawback with the empirical models is that they provide little physical insight.
Finally, an empirical model can only handle a very limited number of factors, typically fewer than three.
However, it is often difficult to apply these models to real cases (e.g., well logs).
(“A new velocity model for clay-sand mixtures,”Geophysical Prospecting 43, 91-118 (1995); and “A physical model for shear-wave velocity prediction,”Geophysical Prospecting 44, 687-717 (1996)) But the model does not handle the anisotropy effect.
But, their model is limited to pure shales only, not applicable to sandy shales or shaly sands.
This greatly limits the applicability of their model since sedimentary rocks are made of not only shales but also other rock types, such as sandstone, siltstone, limestone, etc.
Also, Hornby's approach is valid for high frequencies only and it is, therefore, always Gassmann-inconsistent.
Unfortunately, the proposed method does not work for the anisotropic case.
It is highly questionable if the approach is valid in shaly sands, in which micro-pores tend to be water-wet due to the capillary effect.

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[0045]The present inventive method was applied to log analysis and seismic modeling, and significant improvements were observed in S-wave prediction and well-to-seismic gather tie. FIGS. 5B-E compare the measured (darker curve) S-wave 52 (FIG. 5B) and density 54 (FIG. 5C) logs with the calculated (53, 55) using the traditional isotropic rock physics model. A consistent bias can be observed between the measured and the calculated over the sand intervals 51, corresponding to low values of Vsh, the shale volume fraction, which is shown as a function of depth in FIG. 5A. This was interpreted as the effect of stress-induced anisotropy in unconsolidated sands. FIGS. 5D (S-wave) and 5E (density) show that the present inventive method dramatically improves agreement between the calculated results 57, 59 and the measured data 52, 54 in the sand intervals 51.

[0046]In more detail, shear-wave and density logs can be predicted from the P-wave log (not shown) and the shale volume (shown in FIG. 5...

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Abstract

Method for constructing an integrated rock physics model that simulates both shale anisotropy and stress-induced anisotropy of clastic rocks. In the model, the total pore volume is divided into three parts according to the estimated shale volume and effective stress: (1) clay-related pores, (2) sand-related pores, and (3) microcracks (mainly in the sand component). The pore space is then partitioned into the clay-related and sand-related pores using a scheme first disclosed by Xu and White in 1995. The model simulates shale anisotropy via the preferred orientation of clay-related pores and stress-induced anisotropy via the preferred orientation of microcracks, which is controlled by the differential stresses. Laboratory measurements or well logs are needed to establish a relationship between crack density and the effective stress.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is the National Stage of International Application No. PCT / US2005 / 039002, filed 27 Oct. 2005, which claims the benefit of U.S. Provisional Patent Application No. 60 / 633,675 filed Dec. 6, 2004.FIELD OF THE INVENTION[0002]This invention relates generally to the field of geophysical prospecting and, more particularly, to rock physics modeling and methods for predicting and relating elastic properties, which affect seismic data and therefore can be deduced from seismic data, and petrophysical properties. Specifically, the invention is a method for constructing a rock physics model of a subterranean region that accounts for anisotropy in the region.BACKGROUND OF THE INVENTION[0003]Rock physics modeling plays a critical role in seismic inversion and interpretation by providing a critical link between petrophysical properties (e.g., porosity, shale volume, water saturation, etc.) and seismic properties (e.g., P- and S-wave veloc...

Claims

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Application Information

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Patent Type & Authority Patents(United States)
IPC IPC(8): G06F7/60G06F17/10
CPCG01V99/00
Inventor XU, SHIYUSALTZER, REBECCA L.KEYS, ROBERT G.
Owner EXXONMOBIL UPSTREAM RES CO
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